تحلیل خطر لرزه‌ای احتمالاتی استان همدان با رویکرد شبیه‌سازی مونت‌کارلو و ارزیابی کفایت آیین‌نامه ۲۸۰۰ در پهنه گسلی نهاوند

The aim of this study is to carry out a detailed Probabilistic Seismic Hazard Assessment (PSHA) for Hamadan Province in western Iran, with particular emphasis on the seismogenic potential of the Nahavand Fault, and to compare the results with the design provisions of Iranian Seismic Code Standard No. 2800. The region lies within the active Zagros fold-and-thrust belt, where complex fault systems and heterogeneous seismicity patterns necessitate site-specific hazard analyses, especially for critical facilities. In this context, we assembled a homogenized earthquake catalog for the period 1990–2025, converted to moment magnitude using empirical relations developed for the Iranian plateau, and delineated seismogenic sources based on neotectonic, geological, and seismological evidence. Special attention was given to the geometry and segmentation of the Nahavand Fault, including its southern termination, where negative flower structures and distributed deformation indicate ongoing transpressional activity.



Ground motion parameters were estimated through Monte Carlo simulation of 50,000 stochastic earthquake scenarios combined with a logic-tree framework to account for epistemic uncertainties in source characterization and ground motion prediction equations (GMPEs). The logic tree incorporates alternative source models for the Nahavand Fault (linear fault source versus areal seismogenic zone), different magnitude–frequency distributions, and multiple regionally compatible GMPEs. For each realization, synthetic ground motion fields were generated and uniform hazard spectra (UHS) were derived at representative sites across the province for return periods of 475 and 2475 years, corresponding to standard and maximum-considered earthquake levels, respectively.



The results for a 475-year return period indicate a distinctly non-uniform spatial distribution of seismic hazard in Hamadan Province. The highest levels are obtained in Nahavand County, where the maximum peak ground acceleration (PGA) reaches 0.466g. This value is approximately 33.1% higher than the upper bound of the “very high relative hazard” zone defined in Standard 2800 for the region. Elevated hazard levels are also observed in areas located in close proximity to the mapped trace of the Nahavand Fault and its adjacent segments, reflecting the dominant contribution of this structure to seismic risk in the southern part of the province. For the 2475-year return period, the estimated spectral accelerations in the short- and intermediate-period ranges at sites situated near the fault trace exceed the corresponding code-based design values prescribed in Standard 2800, particularly for fundamental periods relevant to low- to mid-rise buildings.



These results should, however, be interpreted in light of the assumptions and limitations inherent in site-specific PSHA. The hazard estimates are conditioned on the adopted source geometry, maximum magnitude, recurrence parameters, selected GMPEs, and the quality and completeness of the input data. Epistemic uncertainties are explicitly treated through the logic-tree approach, while aleatory variability is represented by the dispersion in ground motion predictions; nevertheless, residual model uncertainty remains. Consequently, the findings are not intended to imply a general inadequacy or invalidation of the national seismic zonation framework or the philosophy of Standard 2800. Rather, they underscore the importance of conducting detailed local hazard assessments for critical structures, lifelines, and strategic facilities located in the vicinity of major active faults such as the Nahavand Fault. In such contexts, reliance solely on broad-scale national zoning maps may underestimate the true site-specific hazard, whereas complementary PSHA can provide more realistic input motions for performance-based seismic design and risk mitigation.



Keywords: Probabilistic Seismic Hazard Assessment (PSHA); Monte Carlo simulation; logic tree; Nahavand Fault; Hamadan Province; Standard No. 2800; uniform hazard spectrum.